1. Technical Field
The present invention relates to an acoustic transducer that converts acoustic vibrations into electrical signals, or converts electrical signals into acoustic vibrations, and more particularly, to an acoustic transducer such as an acoustic sensor or a speaker manufactured using MEMS technology.
2. Related Art
FIG. 1 is a cross-sectional view showing a portion of a conventional acoustic sensor manufactured using MEMS technology. In an acoustic sensor 11, a diaphragm 14 (vibration electrode film) having conductivity is provided above an upper surface of a silicon substrate 13. The silicon substrate 13 has a back chamber 12 vertically penetrating therethrough. The top of the back chamber 12 is covered by the diaphragm 14. Further, a dome-shaped protective film 15 is formed above the upper surface of the silicon substrate 13, enclosing the diaphragm 14. The protective film 15 is formed with a fixed electrode film 16 at a position facing the diaphragm 14. The diaphragm 14 and the fixed electrode film 16 constitute a capacitor for converting acoustic vibrations into electrical signals. Multiple acoustic holes 17 are formed in the protective film 15 and the fixed electrode film 16 to allow acoustic vibrations (sound) to pass through them.
In the acoustic sensor 11 shown in FIG. 1, the diaphragm 14 is formed in parallel with the upper surface of the silicon substrate 13 in a region where the silicon substrate 13 and the diaphragm 14 face each other. In particular, in a direction parallel to the upper surface of the silicon substrate 13 and orthogonal to an edge of a top opening of the back chamber 12, the height of a gap between the silicon substrate 13 and the diaphragm 14 (hereinafter, the gap is referred to as a vent hole 18) is uniform. Such an acoustic sensor is disclosed, for example, in Patent Document 1.
A vent hole of an acoustic sensor serves as an acoustic resistance to acoustic vibrations entering through acoustic holes and passing to a back chamber, and has an important function for ensuring sensitivity in the bass range. On the other hand, air in the vent hole has characteristics as a viscous fluid, and thus the vent hole also functions as a noise (thermal noise) source.
Noise in the vent hole is mainly caused by a mechanical resistance due to the viscosity of air present in the gap (vent hole) between an edge portion of a diaphragm and an upper surface of a silicon substrate (this is called a film damping effect.). Specifically, when the diaphragm tries to move in a direction to be taken off from the substrate (upward), the viscosity of air in the vent hole generates a resistance hindering the upward movement of the diaphragm. Conversely, when the diaphragm tries to move in a direction to be pressed against the substrate (downward), it generates a resistance hindering the downward movement of the diaphragm. Noise caused by a mechanical resistive component at this time constitutes noise in the vent hole.
In the acoustic sensor 11 shown in FIG. 1, in an attempt to reduce generation of noise in the vent hole 18, the diaphragm 14 may be moved away from the upper surface of the silicon substrate 13 to increase the height H of the vent hole 18 like the diaphragm 14 shown in solid lines in FIG. 2A. Alternatively, like the diaphragm 14 shown in solid lines in FIG. 2B, the edge of the diaphragm 14 may be retracted toward the center to shorten the overlap length between the diaphragm 14 and the upper surface of the silicon substrate 13 (width W of the vent hole 18).
However, either when the height H of the vent hole 18 is increased or when the width W of the vent hole 18 is shortened, the acoustic resistance of the vent hole 18 is reduced. Therefore, acoustic vibrations are likely to leak into the back chamber 12 through the vent hole 18, lowering the sensitivity of the acoustic sensor 11 in the bass range. FIG. 3 is a graph showing the sensitivity of the acoustic sensor, with a horizontal axis representing the frequency of acoustic vibrations (vibration frequency), with a vertical axis representing the sensitivity. A curve shown in a dashed line in FIG. 3 represents the sensitivity-frequency characteristics (hereinafter, referred to as frequency characteristics) when the diaphragm 14 is in a position shown in dashed lines in FIG. 2A or FIG. 2B. When the height H of the vent hole 18 is increased as shown in solid lines in FIG. 2A, the sensitivity of the acoustic sensor decreases in the bass range (low audio frequency range) like the frequency characteristics shown in a solid line in FIG. 3. When the width W of the vent hole 18 is shortened as shown in solid lines in FIG. 2B, the sensitivity of the acoustic sensor decreases in the bass range like the frequency characteristics shown in the solid line in FIG. 3. That is, an attempt to reduce noise in the acoustic sensor causes a decrease in sensitivity in the bass range, narrowing a flat range in the frequency characteristics.
On the contrary, in order to provide excellent frequency characteristics of the acoustic sensor (that is, in order to widen the flat range in the frequency characteristics), the diaphragm 14 may be moved closer to the upper surface of the silicon substrate 13 to decrease the height H of the vent hole 18 to increase the acoustic resistance in the vent hole 18. Alternatively, the width W of the vent hole 18 may be lengthened to increase the acoustic resistance. However, in these cases, noise generated in the vent hole 18 increases, degrading the S/N ratio of the acoustic sensor.
Thus, in the conventional acoustic sensor, achieving a high S/N ratio by reducing noise and achieving almost flat frequency characteristics also in the bass range are in a trade-off relationship. It has been difficult to achieve both of them. FIG. 4 is a graph showing a relationship between the S/N ratio (vertical axis) and the roll-off frequency in an acoustic sensor as in FIG. 1. Generally, a roll-off frequency fr is a frequency at a point where the sensitivity decreases by −3 dB compared to the sensitivity at a frequency of 1 kHz. As the roll-off frequency fr becomes smaller, the flat range in sensitivity extends toward the bass range, providing excellent frequency characteristics. FIG. 4 shows that when the roll-off frequency is decreased, the S/N ratio decreases, and when the S/N ratio is increased, the roll-off frequency increases, reducing the sensitivity in the bass range.
Next, FIG. 5A is a cross-sectional view showing a portion of another conventional acoustic sensor manufactured using MEMS technology. FIG. 5B is an enlarged perspective view showing a portion of a diaphragm used in the acoustic sensor in FIG. 5A. In an acoustic sensor 21, a plurality of stoppers 22 is provided on a lower surface of a diaphragm 14. The stoppers 22 prevent an edge portion of the diaphragm 14 from sticking to an upper surface of a silicon substrate 13 and becoming immovable. Such an acoustic sensor is disclosed, for example, in Patent Document 2.
According to the acoustic sensor 21, the distance between the stoppers 22 and the upper surface of the silicon substrate 13 is smaller than the distance between a lower surface of the edge portion of the diaphragm 14 and the upper surface of the silicon substrate 13. Thus, it seems that the stoppers 22 can increase acoustic resistance to increase the sensitivity of the acoustic sensor 21 in the bass range. However, the stoppers 22 are intended to prevent the diaphragm 14 from sticking to the silicon substrate 13, and are formed in a thin pillar shape and provided only sparsely at intervals. Therefore, the stoppers 22 do not have an effect of preventing acoustic vibrations from passing through the vent hole 18. There is no effect of improving the sensitivity of the acoustic sensor 21 by increasing the acoustic resistance.